I used to work in powder coating - just a couple of tips:
- make sure your coater has a zinc phosphate system, not iron. Alloys and/or aluminum will not etch without zinc. This will GREATLY help with adhesion.
- go ahead and pay extra for a clear coat over the color - it's well worth it.
- use polyester or acrylic based powder with UV protection - this will help it weather better - won't get that ashy coating after a while like some of the cheaper powders do.